1. Field of the Invention
The present invention relates to an image heating device, or more specifically, to an image heating device which is preferably applicable to fixing of a non-fixed image used for an electrophotographic device or electrostatic recording device, etc., through heating.
2. Description of the Related Art
As this type of image heating device, there is a conventional proposal on an image heating device using electromagnetic induction. One example of this is an image heating device disclosed in the Unexamined Japanese Patent Publication No.HEI 10-232575.
As shown in FIG. 1, an image heating device 1 is constructed of a fixing member 2 and a pressure roller 3. The fixing member 2 is provided with a stay 4 and a fixing film 5 is attached to this stay 4 in such a way that it is rotatable around the stay 4 in the direction indicated by an arrow. The stay 4 contains an exciting coil 6 and an induction heating plate 7. The exciting coil 6 consists of a core 6a made of a ferromagnetic substance and a winding 6b which is wound around the core 6a. In this way, by passing a high-frequency AC through the winding 6b and generating an alternating field, it is possible to generate an eddy current in the induction heating plate 7 and thereby heat the induction heating plate 7. Furthermore, a temperature sensor 8 is provided close to the induction heating plate 7. The high-frequency current is controlled according to the temperature detection result obtained through the temperature sensor 8 and the temperature of the induction heating plate 7 is set to a desired value.
In the image heating device 1, with the induction heating plate 7 being heated, the pressure roller 3 rotates while contacting the induction heating plate 7 under pressure through the fixing film 5 and carries a recording sheet into a nip section of the fixing film 5 which rotates driven by the pressure roller 3. As a result, toner on the recording sheet is heated and pressurized and thereby fixed to the recording sheet.
In addition to such a configuration, the image heating device 1 is provided with a vibration absorption member 9 between the induction heating plate 7 and exciting coil core 6a. This prevents image disturbance caused by vibration due to electromagnetic induction.
However, in the above described conventional configuration, the vibration absorption member 9 is placed in an area which is directly heated by electromagnetic induction. For this reason, a highly heat-resistant material needs to be used for the vibration absorption member 9, which increases the cost of the device.
Furthermore, the exciting coil 6 and vibration absorption member 9 are placed inside the fixing film 5 which is heated to a high temperature. For this reason, these components are required to have high heat resistance.
Furthermore, the pressure from the fixing nip is received by the thin induction heating plate 7. This causes a great pressure to act on the vibration absorption member 9 and exciting coil 6. To withstand this great pressure, the vibration absorption member 9 needs to be placed over the entire width, which requires the use of a large amount of the costly vibration absorption member 9.
Moreover, because of the great pressure acting thereupon, it is not easy for the thin vibration absorption member 9 to absorb vibration sufficiently. In the case of insufficient vibration absorption, vibration produced when a high-frequency current passes through the exciting coil 6 is transmitted to the fixing film 5, which disturbs a toner image on the recording sheet in the fixing nip section and may change the rotation speed of the fixing film 5, thus generating jitter on the image in extreme cases.
This tendency may become more noticeable when a high-frequency current not lower than approximately 50 kHz is passed to heat a copper or aluminum material having low magnetic permeability and resistivity.
Furthermore, since it is difficult to fix the exciting coil 6 to the induction heating plate 7 firmly, there is a problem that the positioning reliability deteriorates. As a result, when the distance between the exciting coil 6 and induction heating plate 7 fluctuates, the magnetic coupling condition between the exciting coil 6 and induction heating plate 7 changes, which makes it difficult to perform stable power supply control and prevents accurate temperature control. Consequently, the toner fixing state changes, causing an uneven luster or fixing defect.